Hexafluoroethane (CF3CF3) is used, for example, as a cleaning or etching gas for semiconductors. With respect to the production process of CF3CF3, various methods have been heretofore known. Examples thereof include:
(1) a method of fluorinating dichlorotetrafluoroethane, chloropentafluoroethane or the like by using hydrogen fluoride in the presence of a fluorination catalyst, and
(2) a method of directly fluorinating tetrafluoroethane and/or pentafluoroethane by using a fluorine gas.
However, for example, when the method of (1) above is used, compounds originated in starting materials or compounds newly produced by the reaction are contained as impurities in the produced CF3CF3. Among these impurities, chlorine-containing compounds are difficult to separate from CF3CF3 and become a problem.
Also, when the method of (2) above is used, compounds originated in the starting materials or compounds newly produced by the reaction are contained as impurities in the produced CF3CF3.
Also, in these impurities, chlorine-containing compounds which are difficult to separate from CF3CF3 become a problem. To solve this problem, the reaction with a fluorine gas may be performed after purifying the starting material to reduce the chlorine-containing compounds contained therein, but industrial practice of conventionally known purification methods is difficult, in many cases.
Examples of the chlorine-containing compounds contained in CF3CF3 produced by the above-described method include compounds such as chlorodifluoromethane, chlorotrifluoromethane, dichlorotetrafluoroethane, chloropentafluoroethane, 1-chloro-2,2,2-trifluoroethane, 1,1-dichloro-2,2,2-trifluoroethane and 1-chloro-1,2,2,2-tetrafluoroethane.
Among these chlorine-containing compounds, chlorotrifluoromethane forms an azeotropic mixture with CF3CF3 and separation of this compound is difficult. As for the method of purifying this chlorotrifluoromethane-containing CF3CF3, for example, U.S. Pat. No. 5,523,499 describes a process of contacting CF3CF3 containing trifluoromethane (CHF3) or chlorotrifluoromethane (CClF3) as impurities with an adsorbent such as activated carbon or molecular sieve to adsorb and thereby remove the impurities.
According to the purification method using such an adsorbent, in the case of a stationary operation, the adsorbent must be regenerated approximately at regular intervals and equipment therefor is necessary, though this may vary depending on the impurity content. Also, a large amount of gas may be continuously treated by using, for example, a method of disposing two adsorption tower units and alternately changing over the operation between the step of adsorbing impurities and the step of regenerating the adsorbent, but the chlorotrifluoromethane adsorbed and thereby removed cannot be released as it is into air and must be treated by some method because this is one of specified the fluorocarbons which are thought to deplete the ozone layer.
Furthermore, for example, chloropentafluoroethane (CF3CClF2) which is one of starting materials for the production of CF3CF3 does not form an azeotropic mixture with CF3CF3 but, when remaining in the product, this compound is difficult to separate. For purifying this chloropentafluoroethane, for example, Japanese International Application Domestic Publication No. 9-508626 describes a purification method, using extractive distillation, of adding an extractant. In this purification method using extractive distillation, the extractant added must be recovered through distillation by further using a distillation column, which causes a problem such as increase in the equipment or energy cost, and complete removal of the impurity chloropentafluoroethane can hardly be attained.
Accordingly, chlorine-containing compounds are contained as impurities in the step of producing CF3CF3 and in the gas produced. The objective CF3CF3 is usually recovered as a low boiling component from the top of a distillation column and further passed though a purification step to obtain high-purity CF3CF3 as a product. On the other hand, the chlorine-containing compounds contained as impurities are separated as a high boiling component, that is, as a bottom component of the distillation column. In this bottom component, CF3CF3 is sometimes contained in a concentration of about 90 to 97 mol %. For example, in the method of producing CF3CF3 by a reaction of pentafluoroethane with a fluorine gas, the chlorine compounds contained in raw materials do not participate in a displacement reaction with the fluorine gas and therefore, are gradually concentrated in the bottom of the distillation column. According to the degree of concentration, for example, a step of discarding the chlorine-containing compounds by a burning treatment or the like is necessary. However, as described above, high-concentration CF3CF3 is contained in this bottom component and it is required to recover the CF3CF3 by a purification operation.